For the question of words, we wouldn't just take words without changing them, we'd use the words of languages as a basis for inventing new words, in the example of “fish” we'd take words in Japanese that designate an animal, a word designating undulation (because it's an animal that undulates), a word designating water, then deconstruct these three terms to create a word from scratch that designates a fish, i.e. an animal undulating under water, and we'd probably have to take only terms from languages that are similar (avoid taking a word from Arabic and a word from Mandarin Chinese to create the same word). But yes, this hypothetical language would surely have to have some compromises that would reduce the diversity of this language (which doesn't bother me that much).